The present invention relates to a screw rotor machine for a working fluid comprising a pair of intermeshing rotors having helical lands and intervening grooves, and a casing with a working space generally comprised of two intersecting bores, each enclosing one of the rotors, said casing having a low pressure end section, a high pressure end section carrying bearings for the rotors and an intermediate barrel section, said barrel section being connected to at least one of said end sections by a detachable joint, said high pressure end section being provided with a bearing chamber separated from the working space by an end wall provided with holes, each freely surrounding a cylindrical projection of a rotor.
Due to the necessary manufacturing and running clearances in a practical machine a leakage path exists for the high pressure gas to escape from the working space to the bearing chamber in the high pressure end section. The gas thereby leaks from a working chamber radially through the axial clearance between the end surfaces of the rotors and the inner surface of the high pressure end wall to the cylindrical rotor projections. From there the leaking gas flows axially through the radial clearance between each cylindrical rotor projection and the related hole in the end wall into the bearing chamber. To avoid that this gas leakage builds up a pressure in the bearing chamber equivalent to the discharge pressure, as such a pressure would exert an additional axial load to the rotors, provisions are usually taken for allowing the gas to escape from the bearing chamber. Thus it is important to minimize the gas leakage from the working space to the bearing chamber in order to reduce the decrease of the eficiency of the machine resulting from the losses of high pressure gas.
Owing to the deflection of the rotors resulting from radial forces acting thereon, there must be a certain clearance between each rotor projection and the related hole in the end wall to avoid the risk for seizing therebetween. To reduce the leakage by diminishing the clearances to a minimum corresponding to said necessary raunning clearances not only requires narrow tolerances for the cylindrical rotor projections and the related holes in the end section but also necessitates a precise positioning of the low and high pressure end sections relative to each other. When mounting an end section to the barrel section, the end section, due to the play between the fastening bolts and the respective holes in the end section, is movable to a certain degree before the bolts are drawn tight. The end section therefore has to be displaced and angularly adjusted within the limits of said play in order to adjust the position thereof so that the holes in the end section receiving the rotor projections will be aligned with those of the other end section. When the end section is adjusted to the proper position it has to be fixed in this position by means of guiding pins until the bolts are drawn tight. With this extensive adjusting procedure that is necessary for diminishing the clearance to a minimum the manufacturing costs will be increased.
Different kinds of contact seals therefore have been used to prevent gas leakage from the high pressure end of the working space to the bearing chamber in the high pressure end section. See for example the article "Gleitringsdichtungen fur Schraubenmashinen - Stand der Technik" by K. H. Victor published in VDI Berichte 521 entitled "Schraubenmaschinen", pages 49-76, VDI-Verlag GmbH, Dusseldorf, 1984. Many of such contact seals are of complicated construction with sustantial axial length and require supply of oil to cool the seal. The distance between the rotor body and the radial bearings is thereby increased to make space for the sealing arrangement which results in a greater deflection of the rotors. Furthermore, the contact seals give rise to friction losses which negatively affects the efficiency of the machine.
Another way of solving this problem is to use a sealing liquid for blocking the gas leakage. An example of this kind of sealing is disclosed in U.S. Pat. No. 3,462,072 showing a screw compressor in which oil having a pressure exceeding the discharge pressure is supplied from a pressure oil source through a channel in the high pressure end section to annular grooves therein, each of those grooves surrounding one of the shafts. this oil blocks the flow of working fluid along the shafts and thus acts as a sealing liquid between the working space and the bearing chambers in the high pressure end section. A fraction of the oil flows along the shafts into the working space, but most of the oil flows along the shafts in the opposite direction to the bearing chambers and lubricates the bearings. The bearing chambers are drained through a channel in the casing to an opening in the barrel wall of the compressor.
Also, this method of sealing has disadvantages. The oil drained to the compressor will have a considerably higher temperature than the temperature of the working fluid in the working chamber to which the oil is returned. The contact between the working fluid and the oil therefore results in heating of the working fluid which decreases the volumetric efficiency. Furthermore the oil has to be accelerated to the tip speed of the rotor lobes which also consumes power. Another drawback with a sealing arrangement of this kind is that the oil leaking out into the working space exerts an additional axial load to the rotors.
The object of the present invention therefore is to prevent gas leakage along the rotor projections at the high pressure end of a screw rotor machine as defined, in a way not entailing the disadvantages connected to the solutions according to known techniques.